7.5.38.1 Creating hash tables

Create and answer a new hash table with equal-proc as the
equality function and hash-proc as the hashing function.

By default, equal-proc is equal?. It can be any
two-argument procedure, and should answer whether two keys are the
same for this table’s purposes.

My default hash-proc assumes that equal-proc is no
coarser than equal? unless it is literally string-ci=?.
If provided, hash-proc should be a two-argument procedure that
takes a key and the current table size, and answers a reasonably good
hash integer between 0 (inclusive) and the size (exclusive).

weakness should be #f or a symbol indicating how “weak”
the hash table is:

#f

An ordinary non-weak hash table. This is the default.

key

When the key has no more non-weak references at GC, remove that entry.

value

When the value has no more non-weak references at GC, remove that
entry.

key-or-value

When either has no more non-weak references at GC, remove the
association.

As a legacy of the time when Guile couldn’t grow hash tables,
start-size is an optional integer argument that specifies the
approximate starting size for the hash table, which will be rounded to
an algorithmically-sounder number.

By coarser than equal?, we mean that for all x and
y values where (equal-procxy),
(equal? xy) as well. If that does not hold for
your equal-proc, you must provide a hash-proc.

In the case of weak tables, remember that references above
always refers to eq?-wise references. Just because you have a
reference to some string "foo" doesn’t mean that an association
with key "foo" in a weak-key table won’t be collected;
it only counts as a reference if the two "foo"s are eq?,
regardless of equal-proc. As such, it is usually only sensible
to use eq? and hashq as the equivalence and hash
functions for a weak table. See Weak References, for more
information on Guile’s built-in weak table support.